It's a shame that the undescended testicle increases the risk so it's good that you're tuned into that. It's also good that your son feels he can be open with you about issues having to do with private parts. Most kids aren't. Since he's approaching puberty, it's probably a good idea to arrange for a complete physical exam. With the UT history, it's also a good idea to see if his doctor can order annual ultrasounds. That's the only reliable way to detect a possibly malignant solid mass versus a variety of benign things that can crop up. Hopefully the lump he found turns out to be nothing.
Good luck and let us know how it goes,